Although bloodstream infections (BSI) are frequently attributed to bacterial pathogens, fungal infections caused by Candida species actually represent the fourth leading cause of BSI in the United States and present a specific risk for immune compromised patients (Morrell et al, Delaying the Empiric Treatment of Candida Bloodstream Infections until Positive Blood Culture Results Are Obtained: a Potential Risk Factor for Hospital Mortality, Antimicrob. Agents Chemother., 49:3640-3645 (2005); Pfaller et al, Epidemiology of Invasive Mycoses in North America, Crit. Rev. Microbiol., 36:1-53 (2010); Falagas et al, Relative frequency of albicans and the various nonalbicans Candida spp among candidemia isolates from inpatients in various parts of the world: a systematic review, Int. J. Infect. Dis., 14:e954-e966 (2010)). The incidence of candidiasis has increased dramatically over the previous two decades, resulting in significant morbidity, mortality (40-49%6) and increased healthcare costs. Among the Candida spp., C. albicans is the primary cause of BSI (45.6%), followed by C. glabrata (26.0%) (Horn et al, Clinical characteristics of 2,019 patients with candidemia: data from the PATH Alliance Registry, Clin. Infect. Dis., 48:1695-1703 (2009)). However, C. glabrata represents an increasing threat as studies show that while C. glabrata accounted for 18% of BSI candidemia between 1992-2001, that fraction rose to 26% in the time period 2001-2007.
The administration of effective empirical therapy for fungal BSI significantly reduces mortality (27% vs 46%) (Parkins et al, Adequacy of empirical antifungal therapy and effect on outcome among patients with invasive Candida species infection, J. Antimicrob. Chemother., 60:613-6186 (2007)). Unfortunately, however, there is often a significant delay in the correct diagnosis of candidiasis, identification of the species and start of therapy to which the strain is sensitive. While C. albicans remains relatively sensitive to azoles, flucytosine and echinocandins, C. glabrata exhibits decreased sensitivity for fluconazole, with evidence of cross-resistance to other azoles such as voriconazole (Borst et al, Rapid Acquisition of Stable Azole Resistance by Candida glabrata Isolates Obtained before the Clinical Introduction of Fluconazole, Antimicrob. Agents Chemother., 49:783-787 (2005); Magill et al, Triazole cross-resistance among Candida spp.: case report, occurrence among bloodstream isolates, and implications for antifungal therapy, J. Clin. Microbiol., 44:529-535 (2006)); 11% of fluconazole-resistant strains are reportedly now also resistant to echinocandins (Pfaller et al, Decreased Susceptibility and Resistance to Echinocandins among Fluconazole-Resistant Bloodstream Isolates of Candida glabrata, J. Clin. Microbiol., 50:1199-1203 (2012)). The increased incidence of C. glabrata as a causative agent of candidiasis along with the increasing drug resistance in this strain makes new antifungals that target C. glabrata a clear priority. However, an ideal agent would target both C. albicans and C. glabrata as C. albicans infections continue to be a major health risk and the two are difficult to distinguish in a clinical setting.
Targeting the essential enzyme dihydrofolate reductase (DHFR) has proven to be an effective strategy for both prokaryotic (eg. trimethoprim) and protozoal (eg. pyrimethamine) pathogens, but is not widely used clinically in the treatment of invasive fungal infections. DHFR plays a critical role in the turnover of folate cofactors; effective inhibition of DHFR produces a blockade in thymidine synthesis leading to “thymineless” death. As humans are also dependent on active DHFR, it is important that there is selective inhibition of the pathogenic enzyme. Fortunately, there are several important active site differences between human and Candida species that can be exploited for selectivity. It is widely recognized that the development of antimetabolites targeting C. albicans can be complicated by pronounced inconsistencies between target inhibition and antifungal activity. Attempts to study whether the cell wall or membrane permeability affects the uptake of six unrelated antibiotics targeting intracellular proteins failed to derive a direct relationship (Ziegelbauer, A dual labelling method for measuring uptake of low molecular weight compounds into the pathogenic yeast Candida albicans, Med. Mycol., 36:323-330 (1998)). These same inconsistencies have also complicated the development of antifungal antifolates. For example, Kuyper et al hypothesized that molecular weight was inversely related to antifungal activity and pursued the synthesis and evaluation of over 150 low molecular weight analogs; although the effort produced potent, albeit nonselective inhibitors with good antifungal activity, lead optimization of the antifolates against C. albicans was hindered by lack of correlation between enzyme inhibition and antifungal activity, and the researchers concluded that there was no relationship between activity and inhibitor size or lipophilicity but that differences in transport phenomenon could still play an important role in antifungal activity (High-affinity inhibitors of dihydrofolate reductase: antimicrobial and anticancer activities of 7,8-dialkyl-1,3-diaminopyrrolo[3,2-f]quinazolines with small molecular size, J. Med. Chem., 39:892-903 (1996)). More recently, Otzen et al reported a group of potent C. albicans DHFR inhibitors based on a benzyl(oxy)pyrimidine scaffold (Folate-synthesizing enzyme system as target for development of inhibitors and inhibitors combinations against Candida albicans—Synthesis and biological activity of new 2,4-diaminopyrimidines and 4′-substituted 4-aminodiphenyl sulfones, J. Med. Chem., 47:240-253 (2004)). However, these compounds did not exhibit in vitro antifungal activity. After showing that the compounds were not generally susceptible to efflux, the authors of this study also speculated that the compounds were unable to enter C. albicans. 
The present inventors previously discovered new DHFR inhibitors comprising a 2,4-diaminopyrimidine ring with a propargyl linker to an optionally substituted aryl or heteroaryl ring, as disclosed in U.S. Pat. Nos. 8,426,432 B2 and 8,853,228 B2, each of which is incorporated herein in its entirety. The compounds are pyrimidine derivatives that function as DHFR inhibitors. However, additional DHFR inhibitors and, more specifically, antifungal agents targeting Candida albicans and Candida glabrata, are desired.